The present invention relates to the gasification of coal or other carbonaceous materials, either liquids or solids, in an underground location, by in situ conversion including partial combustion and distillation of volatiles.
The production of gaseous products by reacting coal in subterranean deposits with steam and oxygen-containing gas is amply described in issued patents and in technical literature. In a typical operation spaced apart wells are drilled through the overburden to the coal seam, one to serve as an injection well and the other as a production well. By the various methods well known in the art, an underground linking channel is established for gas flow communication from the injection well to the production well. By introduction of air or other oxygen-containing gas and steam through the injection well at elevated temperature, various reactions may ensue, depending on conditions employed, giving rise to vaporization of liquid hydrocarbons and to the production of hydrogen, carbon monoxide, carbon dioxide and possibly methane, as exemplified by the following type reactions: EQU 2C+O2.fwdarw.2CO EQU C+O.sub.2 .fwdarw.CO.sub.2 EQU C+H.sub.2 O.fwdarw.H.sub.2 +CO EQU 2C+2H.sub.2 O.fwdarw.CH.sub.4 +CO.sub.2 EQU CO+H.sub.2 O.fwdarw.CO.sub.2 +H.sub.2 EQU 2C+H.sub.2 O.fwdarw.H.sub.2 +2CO EQU C.sub.x H.sub.y +(x/2)O.sub.2 .fwdarw.xCO+(y/2)H.sub.2
The composition of the gas product obtained as a result of these competing chemical reactions will depend largely upon prevailing temperature at the site of the reaction and to the relative quantities of H.sub.2 O and O.sub.2 there available. Different modes of operation have been proposed as to the injection of the reactant fluids; thus while some prefer to inject oxygen-containing gas and steam simultaneously into the coal strata, others advocate that the injection of the steam and oxygen-containing gas be alternated. Some of the available alternatives in coal gasification are discussed in U.S. Pat. No. 3,978,920.
Instead of spaced apart wells for injection and production respectively, it is also known to employ a single well, wherein an injection tube is provided concentric to an outer casing or bore pipe, as seen, for example in U.S. Pat. Nos. 3,298,434 and 3,856,084. Reactant fluids are introduced to the coal strata through the injection tube and the gaseous reaction products withdrawn in the annulus between the inner pipe and the bore pipe.
In some instances, as seen for example in U.S. Pat. No. 3,999,607, although separate spaced apart wells are employed for injection and production respectively, each of these wells or the injection well alone may be provided with an outer bore pipe or casing and an inner concentric injection tube. In such arrangement oxygen-rich gas may be injected through the inner tube and a moderating fluid such as steam injected into the concentric annulus formed between the inner tube and the outer casing or well wall. The moderating fluid may be injected simultaneously or intermittently with the oxygen to reduce oxidation reaction temperature and may comprise steam, water, N.sub.2 or CO.sub.2 (U.S. Pat. No. 4,026,357).
Other U.S. patents of interest relating to underground coal gasification include: U.S. Pat. Nos. 3,734,184; 3,770,398; and 4,099,567.
In installations wherein an injection well is employed in which oxygen is introduced into the coal strata through an inner tube and the moderating fluid flows down the annulus between the inner tube and the casing, there is the danger of back flow of combustible gas into the annular space with the possible formation of a potentially explosive mixture. Such flow of combustible gas into the annulus could be prevented if the flow rate of the moderating fluid is sufficiently high. The relative flow rates of steam or other moderating fluid to the oxygen flow rate must be set to foster the desired reactions in the combustion zone. Thus, depending upon the relative geometry of the annulus and the injection tube, rates of downward flow of moderating fluid large enough to purge the annulus properly may be too high relative to the coal gasification reaction requirements. The same techniques and problem also exist regarding liquid carbonaceous deposits. This problem is overcome by the present invention, which allows the introduction of moderating gas in sufficient quantities to satisfy the annular purge requirements while at the same time satisfying the requirements of the desired gasification reaction.